Normally, for example, an overload current protection device of this type detects that the current flowing to a 3-phase motor through a contactor exceeds a safe threshold, and cuts off the current to the motor depending on the detection result, which has been realized by making all or a part the current of the motor flow through a bimetallic element. That is, if a current is applied to the switch made of a bimetal, the bimetal is heated depending on the current magnitude, and the motor current exceeds a safe threshold for a predetermined time, then the bimetal is bent by heat and a switch connection point is set in an OFF state, and the current supply to the control input of the contactor is stopped. However, in the system using the switch, it is difficult to adjust a current when the switch enters the OFF state, and there arises the problem that the system is in the misarrangement state for a long time.
To solve the above-mentioned problem, it has been possible to electronically perform the functions which have conventionally been realized by a bimetallic switch. A reliable and easily adjusted device can be provided using electronic equipment. However, since this electronic system requires a complicated circuit, and a number of parts such as a constant voltage power source, etc. are required to operate a contact system by appropriately detecting a current. Furthermore, a current detection transformer (what is called a CT) is used as current detection unit, which causes the problem that a wide current detection range cannot be obtained because the iron core generates magnetic saturation. There also is a method of using a magnetoresistive element. However, since it has low sensitivity, it requires an iron core, thereby failing in obtaining a wide current detection range as in the case of the above-mentioned CT. Additionally, since the magnetoresistive element has a large fluctuation depending on the temperature and a large difference among the elements, and is subject to an influence of disturbance noise, there occurs the problem that high-precision device means requires a high cost.
Therefore, the present invention aims at providing a low-cost and high-precision overload current protection device capable of expanding a current detection range without a constant voltage power source, etc., and without degradation in precision by an environmental characteristic such as disturbance noise, etc. and a change with time.